1. Field of the Invention
The present invention relates generally to the fields of medical devices and tissue imaging. More specifically, the present invention relates to magnetic resonance imaging of skeletal muscle function.
2. Description of the Related Art
Magnetic resonance (MR) imaging is now used in a variety of applications owing to its versatility. In addition to imaging of anatomy, magnetic resonance imaging can be used to assess physiologic function. Examples of such assessments include perfusion and the mechanical function of the heart. In particular, cardiac function is assessed by acquiring images of the heart at specific time points across the cardiac cycle. Proper timing of these acquisitions is achieved through the use of a triggering mechanism based on the electrocardiogram (ECG). The R-wave of the ECG is used to initiate data collection. Data for specific image frames are taken at designated time intervals referenced to the ECG R-wave. Thus, the result is a set of images of the heart as it exists at each selected time point in the cardiac cycle. If the images are played back as a movie loop, the motion of the heart can be observed, and regional contractile deficiencies noted.
A method for quantifying the contractile function of the heart is known as radiofrequency (RF) tagging. In this method, image data readout is preceded by a composite radiofrequency excitation that produces a series of dark parallel lines in the image. These lines result from the selective saturation of tissue within the field of view (FOV). In cardiac imaging, this excitation would be delivered on the R-wave trigger (i.e. at end diastole). Since material points in the tissue have been saturated, the lines are seen during image playback to move with the tissue as the heart contracts. Two such excitations can be used on the R-wave trigger to produce a grid of lines as seen in FIG. 1. An important feature of this method is that such images can be analyzed using automated techniques to track the tag line motion, and thus produce maps of strain and shear, as well as strain and shear rates.
Examples of cardiac function images using radiofrequency tagging are shown in FIG. 1. These images show the left ventricle in cross-section midway between the base and the apex. The image frame on the left shows the heart at end diastole. This frame corresponds to a very short interval following the ECG R-wave, and shows the tag grid that is produced by the twin composite parallel line excitations. The image frame on the right shows the heart at end systole, corresponding to 300 ms following the ECG R-wave. It is clear that the tag lines show the movement of material points in the cardiac muscle resulting from ventricular contraction. Automated algorithms can track the motion of the tag lines to produce maps of myocardial strain, shear, and velocity.
In contrast to cardiac muscle activity, skeletal muscle activity does not possess an inherent periodicity. Therefore, skeletal muscle function is assessed by acquiring images corresponding to specific force levels rather than to specific time points. Thus, instead of using a single gating trigger to initiate a string of readout excitations at regular intervals, each image frame of skeletal muscle imaging is individually triggered. The resulting images would illustrate regional contractile function of muscle as a function of applied force in an isometric exercise. This is preferred to simply having a subject exert a force prior to acquisition, as voluntary contractions are not exactly reproducible. By using a force (and/or joint position) gating mechanism, reproducible and force correlated results can be obtained.
The prior art is deficient in the lack of effective and non-invasive means of imaging skeletal muscle function on a three dimensional basis. More specifically, the prior art is deficient in the lack of effective means of magnetic resonance imaging of skeletal muscle function wherein the magnetic resonance imaging scanner responds to force and/or angular position as a stimulus for acquisition. The present invention fulfills this long-standing need and desire in the art.
The present invention provides a gating interface that allows an imaging scanner such as magnetic resonance scanner to respond to specific force level and/or angular position as a stimulus for images acquisition. The present invention also incorporates radiofrequency tagging to quantify the movement of regional skeletal muscle. Specifically, the gastrocnemius and soleus muscles were examined in an isometric plantar flexion exercise to examine the relative recruitment patterns of the gastrocnemius and soleus muscles in order to determine the source of phosphorus metabolite level changes observed in non-localized surface coil spectroscopy studies of exercising muscle.
In one embodiment of the present invention, there is provided a method of imaging skeletal muscle by an imaging scanner which is triggered to produce an image of the skeletal muscle by gating pulses triggered in response to specific force levels and/or joint angular positions. The imaging scanner can be, for example, a magnetic resonance scanner, ultrasound scanner or X-ray CT. In general, the gating pulses are generated by a gating interface consisting of force transducer amplification component, filtering component, timers and multiple comparators which are set with a reference voltage corresponding to a specific force level of interest. Alternatively, a single comparitor can be used with a means of selecting the reference voltage for triggering (voltage corresponding to force and/or joint angular position). For regional quantification, the images can be encoded by radiofrequency tagging.
In another embodiment of the present invention, there is provided a skeletal muscle imaging system, comprising a force transducer to provide a load signal; a magnetic resonance scanner; a gating interface that produces gating pulses to trigger the scanner to produce an image of skeletal muscle at specific force levels and/or joint angular positions; and pulse sequences with appropriate triggering commands to permit radiofrequency tagging of the image of skeletal muscle when a selected force level and/or joint position is achieved. The load signal is force-proportional and/or joint position-corresponding. In general, the gating pulses are generated by a gating interface comprises of force transducer amplification component, filtering component, timers and multiple comparators (or a single comparitor with adjustable threshold) which are set with a reference voltage corresponding to a specific force level of interest.
The present invention further provides methods for analyzing muscle force transmission and/or studying muscle injury by applying the method/system disclosed herein.
Other and further aspects, features, and advantages of the present invention will be apparent from the following description of the presently preferred embodiments of the invention given for the purpose of disclosure.